A number of combustors are configured to enhance combustion by inducing one or more vortices of fuel particles entrained in air. To varying degrees, however, each of these combustors is plagued with problems of variable fuel particle size, uniform residence time, and cooling of the interior surfaces of the combustor.
Fuel particles are typically distributed over a size range inside the combustor. The large-sized particles experience the same residence time in conventional combustors as do smaller particles; the time is often insufficient to completely combust these larger fuel particles except within the peak power range of the combustor. The efficiency of most combustors noticeably decreases outside their peak power ranges.
It is desirable to operate combustors at high pressures to increase the efficiency of the combustors. However, cooling problems increase as the pressure increases since compressed air burns hotter than at atmospheric pressure. Some combustors develop internal temperatures of 4000.degree. F. or more which would melt their surfaces if directly contacted by those temperatures. Typically, the outer surface of the combustor is cooled with air circulating around the combustor before the air is introduced into the combustor. In many combustors, cooling steps are provided which introduce air in a direction parallel to the interior surface of the combustor to induce a blanket of air which insulates the interior surface from the combustion gases. However, often 40% of the air introduced into a combustor is used for cooling and not for combustion. The large volume of air required for cooling causes a poor combustion exit temperature distribution which in turn requires additional cooling of the turbines.
Tanasawa, U.S. Pat. No. 3,808,802, describes a vortex combustor which burns fuel-air mixture in a central, forced vortex zone of a first cylindrical combustion chamber and in the outer natural vortex zone of a second cylindrical combustion chamber. There are a number of differences between combustors as taught by Tanasawa and variable residence time combustors according to this invention, described infra, e.g., control of fuel particle residence time, presence of louvres in the vicinity of primary combustion, control of the combustion vortex, and cooling of internal surfaces.
After fuel particles are combusted within primary and secondary combustion zones in conventional combustors, the combustion gases are cooled in a dilution zone in which air is provided to dilute the combustion gases. When a solid fuel such as coal is burned, ash and other by-product particulates are said from the system using a scroll, also known as a cyclone separator, that is presently positioned downstream of the dilution zone.